(a) Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices and, more particularly, to the improvement of the technique forming a nitride film on the surface of semiconductor wafers by using a nitrifying process.
(b) Description of the Related Art
For achieving a higher integrity of semiconductor devices, the technique for reducing the dimensions of constituent elements in the semiconductor devices has been developed. The area for semiconductor memory elements, which are the main constituent elements in the semiconductor devices, has also been reduced without allowing reduction of the memory capacity. FIG. 10 shows an example of a memory cell in a semiconductor device.
The memory cell formed in a conventional semiconductor device 40 includes a MIS transistor formed on the surface region of a semiconductor substrate 11, and a capacitor 41 connected to and overlying the MIS transistor. The semiconductor device 40 includes an array of such memory cells arranged in a matrix on the semiconductor substrate 11. The capacitor 41 includes a bottom electrode 42 formed within a cylindrical hole 30 penetrating a thick insulation film 29, a capacitor insulation film 43 formed on the inner and top surfaces of the bottom electrode 42, and a top electrode 44 opposing the bottom electrode 42 via the capacitor insulation film 43. The bottom electrode 42 is made of polysilicon, for example, heavily doped with phosphorous for suppression of depletion.
As a measure for achieving a higher capacitance for the capacitor in the memory cell without increasing the occupied area thereof, use of a metal oxide film having a higher permittivity as the capacitor insulation film is being investigated. The metal oxide film should be preferably associated with a silicon nitride film formed on the bottom electrode prior to formation of the metal oxide film for prevention of oxidation of the bottom electrode. The silicon nitride film should have a smaller thickness because the silicon nitride film has a relatively lower permittivity. Such a thin silicon nitride film is formed, for example, by a rapid thermal nitrization (RTN) treatment wherein a lamp anneal is conducted in a nitride-compound-gas atmosphere such as including NH3. The technique using the RTN treatment is described in Patent Publication JP-2003-188178A, for example.
The present inventors manufactured in a research a plurality of sample semiconductor devices including the metal oxide film and a silicon nitride film as a capacitor insulation film, and found that a larger range of variation exists in the capacitor characteristics between the sample semiconductor devices. This variation occurred in the capacitor characteristics including the capacitance and leakage current of the capacitor although the silicon nitride films in those samples were formed in similar process conditions within the same reactor chamber for the RTN treatment.
A further research revealed that if the silicon nitride films in a plurality of semiconductor devices are manufactured by the repeated RTN treatment in an in-line processing in the same reactor chamber, a semiconductor device formed in a later RTN treatment has a tendency of incurring a larger reduction in the film thickness and film quality of the silicon nitride film. It is thus concluded that the variation in the capacitor characteristics as described above resulted from the reduction in the film thickness and film quality of the silicon nitride film in the capacitor. Thus, it is essential to achieve a silicon nitride film having a more uniform film thickness and film quality irrespective of the RTN repetition times in the in-line RTN processing using the same reactor chamber.